A fuel injection device which is usable in an engine to be mounted on a saddle ridden type vehicle such as a motorcycle has an injector provided in the vicinity of an intake port in the intake passage of the engine, a sensor for detecting the position of a crank and the rotational speed of the engine, a sensor for detecting intake air pressure, a sensor for detecting the opening degree of a throttle, and a control unit for controlling the operation of the injector on the basis of outputs of those sensors. In this fuel injection device, the control unit performs a fuel injection process of calculating a fuel injection quantity on the basis of outputs of the sensors and controlling the injector such that the injector injects fuel of the calculated fuel injection quantity.
As fuel injection processes, there are a basic fuel injection process for normal driving and a transient fuel injection process for transient driving such as the time of accelerating. In the basic fuel injection process, in order to calculate a basic fuel injection quantity, a calculation system using intake air pressure and engine rotational speed (a speed density system) or a calculation system using the opening degree of a throttle and engine rotational speed (a throttle speed system) may be used. In a case where the resolution of intake air pressure is higher than the resolution of the opening degree of a throttle, the speed density system is often used to calculate a basic fuel injection quantity. In contrast, in a case where the resolution of throttle the opening degree of a throttle is higher than the resolution of intake air pressure, the throttle speed system is often used to calculate a basic fuel injection quantity. Meanwhile, in the transient fuel injection process, since the opening degree of a throttle is good in responsivity, in order to calculate a transient fuel injection quantity, a system using the opening degree of a throttle and engine rotational speed is often used.
Meanwhile, in Patent Document 1, there is disclosed an engine control device which sets a fuel injection quantity for acceleration on the basis of engine rotational speed and an intake air pressure difference.
Patent Document 1: WO 2003/038261
By the way, if a calculation system using intake air pressure and engine rotational speed is used with respect not only to basic fuel injection quantities but also to transient fuel injection quantities, it becomes unnecessary to acquire the opening degree of a throttle. Therefore, it is possible to remove a throttle sensor from an engine, and it is possible to reduce the size and manufacturing cost of the engine. For this reason, in a transient fuel injection process, it is required to calculate a transient fuel injection quantity on the basis of intake air pressure and engine rotational speed.
In the transient fuel injection process, it is required to implement high responsivity of an engine, for example, by immediately performing transient fuel injection in response to a driver's driving operation on a motorcycle. However, in the transient fuel injection process, in a case of calculating a transient fuel injection quantity on the basis of intake air pressure and engine rotational speed, it is not easy to implement high responsivity of the engine.
That is, in general, in a basic fuel injection process of calculating a basic fuel injection quantity on the basis of intake air pressure and engine rotational speed, detection of intake air pressure is performed in an intake stroke or in a compression stroke, and basic fuel injection of the basic fuel injection quantity calculated on the basis of the result of the intake air pressure detection is performed in an exhaust stroke or in the intake stroke of the next cycle. Therefore, between a driver's driving operation and performance of basic fuel injection based on a basic fuel injection quantity changed in response to the corresponding driving operation, a time lag occurs. As a result, in the basic fuel injection process of calculating a basic fuel injection quantity on the basis of intake air pressure and engine rotational speed, the responsivity of the engine to a driving operation is low. Therefore, in a case of using the basic fuel injection quantity calculation method of the basic fuel injection process to calculate a transient fuel injection quantity in a transient fuel injection process, it is difficult to implement high responsivity of the engine. For this reason, as a method of calculating transient fuel injection quantity on the basis of intake air pressure and engine rotational speed, a new method different from the basic fuel injection quantity calculation method of the basic fuel injection process as described above needs to be devised. However, this is not easy.
In Patent Document 1, an acceleration state is detected on the basis of an intake air pressure difference. In a case where an acceleration state is detected, a fuel injection quantity for acceleration is determined on the basis of engine rotational speed and the intake air pressure difference, and fuel injection of the fuel injection quantity for acceleration is immediately performed such that an acceleration feeling intended by the driver can be obtained.
However, in Patent Document 1, with respect to calculation of a fuel injection quantity for acceleration according to engine rotational speed and an intake air pressure difference, it is just disclosed that a fuel injection quantity for acceleration is calculated from a three-dimensional map, and the content of the three-dimensional map is not disclosed. For this reason, from the disclosure of Patent Document 1, whether it is possible to calculate an accurate transient fuel injection quantity according to a driving operation is not apparent, and it is not easy to generate a three-dimensional map for implementing calculation of an accurate transient fuel injection quantity.